This invention relates to a process and apparatus for the direct reduction of iron ores in a reduction furnace charged from above, such as a shaft furnace, wherein hot reducing gas is blown into a reduction zone in the furnace containing a packed bed burden of ore and waste gas or exhausted top gas is removed from the furnace above the reduction zone. Such a process is known from German published patent application No. 17 83 180 in which exhausted reducing gas is withdrawn from the reducing zone, passed through a gas scrubbing operation and a water removal operation, enriched with methane and reintroduced into the lowest section of the shaft furnace, which is a cooling zone, in which it comes into contact with hot reduced pellets. The gases move in counter-flow relationship with the downwardly flowing hot pellets, cooling the pellets and heating the gases which rise upwardly into the reduction zone. The hot pellets act as catalysts, thus when the waste gases, or top gases, enter the reduction zone carbon dioxide and methane are converted to carbon monoxide and hydrogen in accordance with the following gas reforming reaction: EQU CO.sub.2 +CH.sub.4 .fwdarw.2CO+2H.sub.2
The reducing gas so formed operates as a reductant for the direct reduction of the iron oxide burden in the reducing zone. The reference also teaches that part of the cooled waste gas may be removed before the methane enrichment procedure and introduced into a middle region of the shaft furnace for temperature control purposes.
U.S. Pat. No. 4,046,557 discloses a shaft furnace direct reduction process, wherein a part of the cooling gas within the furnace flows upwardly from the cooling zone into the reduction zone. Top gas and natural gas can be added to the prepared cooling gas, the gas being reformed to reducing gas in the shaft furnace, in accordance with the following reactions: ##STR1##
A reduction shaft furnace is usually fed with hot reduction gas from the periphery. When natural gas or spent top gas in injected below the hot reduction gas injection plane, for example into the cooling zone, the upwardly rising gas is forced inwardly to the center of the shaft. When large quantities of cooling gas flow upwardly through the center of the shaft furnace, the burden becomes excessively cooled which slows the ore reduction process. Metallization of the sponge iron produced is impaired. The amount of cracking gas which can be produced by the reaction of methane on the hot sponge iron is also reduced.
The invented process will prevent excess localized cooling within the reduction zone, provide a high degree of iron ore metallization and provide generally uniform reduction of the burden throughout its entire cross-section. When natural gas is introduced to the furnace below the reducing zone, either in concentrated form or mixed with cleaned spent top gas, the heat content of the reduced sponge iron burden is more effectively utilized for in situ reforming of the natural gas to reducing gas than it is in the previously known processes.